The present invention relates to devices which couple to process control loops of the type used in industry. More specifically, the invention relates to detection of events in a process control system by monitoring process signals.
Process control loops are used in industry to control operation of a process, such as an oil refinery. A transmitter is typically part of the loop and is located in the field to measure and transmit a process variable such as pressure, flow or temperature, for example, to control room equipment. A controller such as a valve controller is also part of the process control loop and controls position of a valve based upon a control signal received over the control loop or generated internally. Other controllers control electric motors or solenoids for example. The control room equipment is also part of the process control loop such that an operator or computer in the control room is capable of monitoring the process based upon process variables received from transmitters in the field and responsively controlling the process by sending control signals to the appropriate control devices. Another process device which may be part of a control loop is a portable communicator which is capable of monitoring and transmitting process signals on the process control loop. Typically, these are used to configure devices which form the loop.
It is desirable to detect the occurrence of an event in the process control system. Typically, the prior art has been limited to a simple detection techniques. For example, a process variable such as pressure is monitored and an alarm is sounded or a safety shutdown is initiated if the process variable exceeds predefined limits. However, in order to identify what event triggered the alarm, it is necessary to use complex models which are difficult to implement in a process environment where there is limited power and resources for large computations.
A device in a process control system includes an input which receives a process signal. The device includes memory containing nominal parameter values and rules. In one embodiment, a nominal parameter value relates to trained value(s) of the process signal and sensitivity parameter(s). Computing circuitry in the device calculates statistical parameters of the process signal and operates on the statistical parameters and the stored nominal parameter values based upon the stored rules. The computing circuitry provides an event output related to an event in the process control system based upon the evaluation of the rules. Output circuitry provides an output in response to the event output. In one embodiment, the statistical parameters are selected from the group consisting of standard deviation, mean, sample variance, range, root-mean-square, and rate of change. In one embodiment the rules are selected to detect events from the group consisting of signal spike, signal drift, signal bias, signal noise, signal stuck, signal hard over, cyclic signal, erratic signal and non-linear signal.
The device of the present invention includes any process device such as a transmitter, controller, motor, sensor, valve, communicator, or control room equipment.
In one aspect, the statistical parameter is transmitted over the control loop for use by a statistical process control system.